Dawson was consul-general at large from 1922 to 1924 and served as chief instructor at the Department of State's Foreign Service School from 1925 to 1928. He married Agnes Balloch Bready on 8 June 1926.

He served in Mexico as consul-general; was U.S. Minister to Ecuador, Colombia and Uruguay; and U.S. ambassador to Panama and Uruguay during his long career.

After retiring in 1946 he served as advisor on Latin American affairs to the U.S. delegation during the formation of the United Nations, went to Brazil on a special mission with General George Marshall and became the first U.S. ambassador to the Organization of American States.